Mycoplasma fermentans, a species of human mycoplasmas was first described in 1953. It has been detected in and isolated from tissues of AIDS patients and appears to be associated with a fatal necrotic disease. In order to satisfy Koch's postulates for demonstration of pathogenicity, we have inoculated Macaque monkeys intraperitoneally with broth-grown cultures of the original M. fermentans isolate from an AIDS patient, strain incognitum, as well as the reference strain, PG18. The monkeys which received PG18 nine months ago have shown no symptoms suggesting pathogenesis. Preliminary analysis of serum samples showed very little serological response to the mycoplasma. Those monkeys which received strain incognitum, however, all lost weight during the first two to three months following inoculation, even though the veterinary care- takers reported no change in their intake of food. Specific antibody has been detected in the sera from these animals. Nevertheless, the animals have recovered the lost weight and appear well. These studies are continuing with another group of animals and more strains of Mycoplasma fermentans.